This is definitely an abnormal coloration due to disease, infection, or nerve damage, as the fisheries manager quoted above said. Many species of freshwater fish do change color to match their surroundings, but it never happens to one specific part of the body without affecting the adjoining parts of the body. And those fish that do change can and do change back to "normal" very quickly. I've hooked lots of smallmouth bass that looked absolutely black before they struck the lure, but by the time I landed them they had already begun to lighten up to a more normal coloration.

As an artist who paints a lot of fish (I'm the guy that does all the Bass Pro Shops catalog covers), I'm a student of fish coloration. Smallmouth are an interesting case in point. You know those beautiful dark bars and spots on smallmouth that are so characteristic of the species? The dark markings are actually more of a sign of stress or excitement. Having spent lots of time snorkeling in clear Ozark streams watching smallies, in the water just doing their thing, they are usually almost unmarked. But hook one, and the stress of playing it starts the process of making those bars and spots appear, and if you put it on a stringer for a bit, the stress makes the markings very pronounced. Those markings also appear very strongly in fish who are actively spawning.

This was the tax collector of the group. His community was fed up with not being properly represented so they tarred and feathered him. It's a pretty well known aquatic fact that the feathers never stick underwater.